With the demand for smaller and lighter electronic devices, it is expected to realize more highly integrated semiconductor packages.
In addition, there is demand for more reliable semiconductor packages and, as integrated circuits become increasingly miniaturized, electromigration (EM) resistance has become a growing issue and its effects can no longer be ignored.
Electromigration (EM) is a phenomenon in which there is a momentum exchange between electrons moving through an electrical conductor and metal atoms, the metal atoms gradually move, and defects (voids) occur in the shape of the metal. This phenomenon is more likely to occur when the current density is high.
According to theoretical considerations, when the current flowing in the circuit is the same, the current density known to be the square of k when the size (the height and width in the case of a rectangular circuit, the diameter in the case of a column-shaped circuit) is 1/k, and the effect of electromigration appears to increase at an accelerated rate.
The effects of electromigration can sometimes be severe. In the worst case, a portion of the circuit becomes disconnected, and the device rendered completely inoperable.
As a countermeasure, copper, which is less susceptible to electromigration, is used as the wiring material in semiconductor chips produced in the most recent semiconductor manufacturing processes. However, electromigration remains a problem in joints between semiconductor chips and package substrates as the power consumed per unit area of conductor increases because the current density in these joints is higher. Solder (for example, SnAg) is primarily used in joints between semiconductor chips and package substrates. Also, it is important to reduce current density to increase electromigration resistance.
One countermeasure is to use a structure with a copper pillar in the portions where the current crowding is likely to occur. However, when current is applied for a long period of time, voids caused by electromigration occur, and resistance increases.
Moreover, even when copper is used, there are still some effects of electromigration.
Another countermeasure being considered is to use nickel (Ni) as an anti-dispersion layer to inhibit the movement of metal electrons. However, a defect called a black pad sometimes occurs on the organic substrate side. If a Ni barrier is simply used on the chip side, the copper in the pad on the organic substrate side moves (elutes) into the solder, and this has adverse effects such as a rise in resistance.
Patent Literature 1 discloses a technology in which a semiconductor chip is exposed to high temperatures during practical use in order to form the intermetallic compound (IMC) Cu3Sn, which inhibits occurrence of Kirkendall voids.
However, Patent Literature 1 does not mention the thickness of the Cu3Sn in relation to electromigration.
Patent Literature 2 discloses a technology in which electromigration is inhibited by joining components using solder balls containing a metallic core.
Patent Literature 3 inhibits electromigration by equalizing the current in joints via the incorporation of a pattern in the insulating layer between the bumps and pads of a semiconductor chip.
However, Patent Literature 2 and Patent Literature 3 require special solder balls and insulating layers, neither of which will remain viable as the joint pitches of semiconductor chips are further miniaturized.
Patent Literature 4 and Patent Literature 5 describe the formation of Cu3Sn via aging, but the explanation is only related to the hardness and brittleness that are characteristics of Cu3Sn.
While Patent Literature 6 mentions the EM inhibiting effects of heating Cu wiring in an oxidizing atmosphere, it is only on the reference level.
Non-patent Literature 1 explains how the growth of intermetallic compounds (IMC) in the interface between the solder and the under bump metallurgy (UBM) is increased significantly by electromigration (EM) and creates voids.
Non-patent Literature 1 is helpful in providing a theoretical explanation of the creation of compositions such as Cu3Sn and Cu6Sn5 as intermetallic compounds of copper (Cu) and tin (Sn).